Variable resistor



Sept. 2, 1958 F. B. MacLAREN VARIABLE RESISTOR Filed Aug. 8. 1956 2 Sheets-Sheet 1 S'e'pt. 2, 195.8 F. B. MacLAREN vARIABLE REsIsToR maa Aug. a. 195s- 2 Sheets-Sheet 2 assess@ mutante gustaron Frederick E. Maclharen, Huntington, N. Y., assignerl to Acton Laboratories, inc., Acton, Mass.

Appiication August i956, Serial No. 602,727 i il@ Ciaims. (Cl. 2011-56) The present invention relates to variable resistors for electrical circuits, and particularly multiturn resistors which are adapted for use in electronic circuits.

ln recent years many applications have been developed for the use of variablelresistors in which a plurality of shaft rotations are required to vary the resistance of the resistor from maximum to minimum, such resistors being referred to as multiturn resistors. Most of the multiturn resistors that have been used are bulky and require more space than designers desire to use due to the miniaturization trend. For this reason, it is one of the objects of the present invention to provide a multiturn resistor which may be constructed lin relatively small physical sizes.

Another difficulty experienced with the known multiturn electrical resistors is that it is difcult to provide a resistor with a rotational displacement of the resistor shaft to resistance characteristic with the desired function and with the degree of precision required. Generally, rotation of the shaft of the resistor is required to vary the resistance of the resistor linearly with relatively small deviations.- It is a further object of the invention to provide M:yariable resistor which may readily be constructed with a desired characteristic of shaft rotational displacement to resistance, particularly a linear shaft displacement to resistance characteristic with a high degree of precision. A further difficulty experienced in systems employing wire-wound variable resistors or potentiometers results from the limited resolution caused by finite resistance increments produced as the slider traverses the resistance element. Another object of the invention is to provide a resistance element of maximum length and diameter, in order to permit the greatest possible resolution for any specified total resistance value and resistance alloy, Within the space limitations for the complete unit.

A further object of the invention is to provide a guide i for advancing the slider assembly which is independent of the resistance element, thus avoiding any moving engagement of the resistance element except by the slider contact.

A still further object of the invention is to provide guiding means for constraining the slider contact within a track centered along the resistance element.

Another object of the invention is to provide means for eliminating all mechanical backlash between the input shaft rotation and slider contact motion.

Still another object of the invention is to provide means for improving the heat transfer between the resistance element and the housing.

It is a further object of the invention to provide means whereby the electrical angle and position of electrical taps may be readily adjusted to a desired value within close limits.

Electronic equipment is often required to operate under adverse temperature conditions. Both ambient conditions and heat generated by the electronic equipment itself require variable resistors to operate over a wide range of i te temperatures. It is a further object of the present invention to provide a variable multiturn resistor which will maintain its electrical characteristics and mechanical operability over a relatively wide range of temperatures.

A still further object of the invention is to provide a variable multiturn resistor which maintains a high value of voltage breakdown and leakage resistance to the housing over a wide range of altitude, humidity and salt spray conditions encountered in military equipment.

It is also an object of the present invention to provide a variable resistor which incorporates these objects and which is readily constructed, rugged, and inexpensive.

The inventor achieves the objects of his invention by providing a novel construction in which the resistance wire of the resistor is disposed in a helix and a sldable contact abuts the resistance wire and is tracked on a second helix disposed adjacent to the resistance helix. A more complete understanding and additional features of the invention may be had from the following disclosure, particularly when viewed in the light of the drawings, in which:

Figure 1 is an elevational view of a variable resistor constructed according to the teachings of the present invention;

Figure 2 is a sectional view taken along the line 22 of Figure 1;

Figure 3 is a sectional view taken along the line 3 3 of Figure l;

Figure 4 is a sectional view taken along the line 4-4 of Figure 1;

Figure 5 is an elevational view of a subcombination showing the sldable contacts of the resistor;

Figure 6 is a sectional view taken along the line 6 6 of Figure 4; and

Figure 7 is a fragmentary sectional view taken along the line 7-'7 of Figure 6.

The variable resistor illustrated in the figures is a multiple turn Z-gang resistor. This resistor has a cylindrical casing or metal housing l0 which is opened at one end l2 and closed at the other end 14. A narrow slot 16 extends longitudinally along one side of the housing 10, and a terminal strip 18 is attached to the exterior surface of the housing 10 confronting the slot i6. The housing l0 is also provided with a shorter second slot 20 which is disposed Y parallel to the axis of the housing l0 contiguous to the open end 12 thereof, and a second terminal board 22 having two terminals 24a and 24h is attached to the exterior surface of the housing 10 confronting the slot 20.

The open end 12 of the housing 10 is closed by a cap 26 which is press ttted into the housing l0. The end cap 26 has a central aperture 28 aligned with the axis of the housing 10,

The closed end 14 of the housing 10 is also provided with a central aperture 30 which is aligned with the axis of the housing 10. A shaft 32 is coaxially journalled in the housing 10 by bearings 34 and 36 disposed within the apertures 30 and 28, respectively. The bearing 34 in the closed end 14 of the housing 10 is maintained in position by a retaining ring 38 disposed within a groove 40 in the closed end 14.

An electrically insulating sheet 46 of material having a relatively high dielectric strength such as Mylar is located between the housing 10 and a resistance element 5t) in order to increase voltage breakdown and leakage resistr ance element 50 in order to increase voltage breakdown and leakage resistance characteristics, even under adverse humidity and salt spray conditions. A suitable high temperature electrical insulating material 48, having relatively high thermal conductivity and good adhesive qualities, such as filled epoxy resin, is disposed on both sides of sheet 46 and secures resistance element 50 within, but in close proximity to housing 10, so that maximum heat transfer occurs therebetween and maximum coil diameter is achieved. Resistance element 50 comprises helical coils 50A and 50B of resistance wire aligned on a common axis. Coils 50A and 50B are identical except that the number of turns and resistivity of resistance wire used may be varied to produce a desired value of total resistance. It is convenient in the manufacture of this resistor to wind both coils 50A and 50B of their desired resistance value on different portions of the same mandrel, with a sufficient gap to electrically insulate one from the other. Each portion of each turn 52 of the coils 50A and 50B is spaced from the adjacent turns 52, this space being constant and containing cement 48. A second helical coil 54 constructedof electrical insulating material is also cemented to the housing, each of the turns 56 of the electrically insulating coil 54 being cemented by the layer 48 midway between a pair of turns 52 of the first pair of coils 50A and 50B of resistance wire, and each of the turns 56 protruding toward the axis of the housing 10 to a greater distance than the turns 52 of the resistance coils 50A and 50B. The turns 56 of the second 'helical coil 54 also hav'e all portions spaced from the adjacent turns 56 of this coil 54, thereby providing each of the turns 52 of the pair of resistance coils 50A and 50B with a surface 57 confronting the axis of the housing 10 which is free of the layer of cement 48 and other electrical insulation.

A pair of parallel spaced contact rings S8 and 60 are also mounted to the housing 10 adjacent to the open end 12 and coaxially therewith by the layer 48 of cement. These contact rings have grooves 62 which confront the axis of the housing 10 and are adapted to accommodate slidable electrical contacts, as will be hereinafter described. The slip ring 58 is electrically connected to the terminal 24a by a conductor 64 which extends through the slot in the housing 10, and the slip ring 60 is connected by an electrical conductor 66 to the terminal 24b in a similar manner.

One carriage support spool 68 is secured to the shaft 32 adjacent to the closed end 14 of the housing 10, a washer 70 being journalled about the shaft 32 between the bearing 34 and the spool 68 to maintain constant this distance. The carriage support spool 68 is annular and has an annular stem 71 which extends toward the open end 12 of the housing 10, the stem being aixed to the shaft 32 by a suitable adhesive 73 for example. A second carriage support spool 72 is affixed to the shaft 32 near the end cap 26, this second spool 72 having a stem 74 extending toward the closed end 14 of the housing 10 and secured to the shaft 32 by adhesive 75. A washer 76 is journalled about the shaft 32 and abuts the bearing 36 in the end cap 26, and a spring washer 78 is disposed between the washer 76 and the adjacent surface of the second support spool 72 in order to place a spring bias upon the bearings 34 and 36, thus removing all radial play and end play from the shaft 32. Shim washer 42 provides necessary clearance between the inner race of bearing 36 and end cap 26. Varying the thickness of washer 42 also serves to adjust the amount of spring bias on the bearings 34 and 36.

A pair of slider support rods 80 and 82 are secured within apertures 84 and 86 in each of the carriage support spools 68 and 72, the slider support rods 80 and 82 being disposed parallel to the axis of the housing. A rectangular sheet of electrical insulating material 88 is secured to one end of each of the support rods 80 and 82 and the surface of the carriage support spool 68 confronting the closed end 14 of the housing 10, and a similar rectangular sheet of electrical insulating material 90 is attached to the other ends of the slider support rods 80 and 82 and the carriage support spool surface 72 in order to provide increased electrical creapage' distance and thereby increase voltage breakdown from rods 80 and 82 to housing 10, washer 78 and pin 172.

Each of the slider support rods 80 and 82 is provided with a slot 92 which is disposed on a plane generally parallel to "the confronting surface of the housing 10. Slip ring contacts 94 are secured within the slots 92 of the slider support rods 80 and 82, and the contacts 94 are slida'bly disposed in contact with the slip rings 58 and 60. In this manner, electrical contact is provided between terminal 24a and the slider support rod 80, and between terminal 24h and the slider support rod 82.

Each of the slip ring contacts 94 is in the form of a wire constructed of electrically conducting material which is provided with a central loop 98, as illustrated in Figure 7. The slip ring contacts 94 are constructed of spring materials, and the ends of the contacts are provided with outwardly extending hooks 100 which are slidably disposed within the grooves 62 of the slip rings 58 and 60. The loops 98 of the slip ring contacts 94 have a straight portion 102 centrally thereof which is disposed within the slots 92 and secured to the support rods 80 and 82.

A pair of discs 104 and 106 with central apertures 108 and 110 are disposed about the shaft 32 between the spools 68 and 72. The discs 104 and 106 also have bores 112 I and 114, and the slider support rods 80 and 82 are slidably disposed within the bores 112 and 114, respectively, so that the discs 104 and 106 are free for translation along the axis of the shaft 32.

The disc 104 is lprovided with a pair of cutouts 116 and 118, each of the cutouts 116 and 118 being a right angle sector disposed on opposite sides of the shaft 32. A slot 120 extends into the disc 104 from the cutout 118 on a plane essentially parallel with the adjacent periphery of the disc 104 and one end of a leaf spring 131 is anchored within the slot 120. The contact assembly 122 is in the form of a strip of spring metal which is electrically conducting. The strip is attached to leaf spring 131, extends over and maintains electrical contact with rod 82, the contact assembly 122 maintaining the support shaft 82 in slidable engagement with the disc 104. At a point just beyond the shaft 82, the contact assembly is provided with a sharp bend 124 and thereafter follows the periphery of the disc 102 to its opposite end. Adjacent to the opposite end of the contact assembly 122 is an outwardly extending hook 126 which rides in slidable engagement with the turns of the electrically conducting coil 50A.`

A slot 128 extends into the disc 104 from the cutout 116 approximately normal to the slot 120. A leaf spring 130 is anchored within the slot 128 and extends outwardly into contact with the shaft 80. The leaf spring 130 and the contact assembly 122 place a spring bias upon the shafts 80 and 82 to maintain no lost motion between rotation of shaft 32 and disc 104.

Between the hook 126 of the contact assembly 122 and the cutout 116,- a single ridge 132 is disposed upon the periphery of the disc 104. This ridge abuts two of the turns 56 of the coil 54 positioning the ridge 132 to confnont one of the turns of the coil 50A. A pair of spaced ridges 134 and 136 are disposed on the periphery of the disc 104 between the cutouts 116 and 118, and these ridges.134 and 136 are slidably disposed on opposite sides of a single turn 56 of the coil 54. As a result, rotation of the shaft 32 translates the disc 104 therealong. It is also to be noted that the contact assembly 122 provides a slidable contact with the resistance coil 50A and with the electrically conducting shaft 82, and hence with terminal 24b.

The disc 106 is also constructed of electrical insulating material, and is provided with a pair of right angle cutouts 138 and 140, similar to the cutouts 116 and 118 in the disc 104. Also a pair of ridges 142 and 144 extend along the periphery of the disc 106 between the cutouts 138 and 140 similar to the ridges 134 and 136. A slot 146 extends into the disc 106 from the cutout 138 adjacent to the bore 114, and a contact assembly 148 is anchored at one end within the slot 146 in a manner 178D is connected to the similar to the contact assembly 122 anchored within the slot 120. The contact assembly 148 is essentially identical to the contact assembly 122, but is inserted into the slot 146 to abut the shaft '80, rather than the shaft 82. Also, a leaf spring 150'-is disposed within a slot 152 extending into the disc 106 essentially normal to the slot 146, and the leaf spring 150 abuts the shaft 82 to provide a spring bias thereon. Further, a ridge 154 extends outwardly from the periphery of the disc 106 between the end of the contact assembly 148 and the cutout 140. The contact assembly 148 has a hook 156 which extends outwardly and is split into two generally parallel portions 156A and 156B. The contact assembly is constructed of non-corroding spring metal, the parallel portions 156A and 156B riding adjacently in contact with the same turn of the resistance coil 50B. The guiding action of two adjacent turns 56 of the inner insulated helix 54 maintains proper alignment of contact portions 156A and 156B with the resistance element 50B, under any anticipated vibration or shock condition. Each of the contact portions 156A and 156B terminates in a semi-circular hook 126, which is shaped in this manner to ride smoothly in both directions over the wires of the resistance element 50B. The double contact arrangement used provides a substantial reduction in the equivalent noise resistance between the slider and resistance element.

Both of the discs 104 and 106 apertures 158 on the side opposite 156, respectively, and a stop shaft 160 is secured within the apertures 158 and protrudes toward the closed end 14 of the housing 10 and the end cap 26. The spool 68 is provided with a slot .162, and the spool 72 is provided with a slot 164 in order to 160 and permit it to be translated therethrough. In addition, the closed end 14 of the housing 10 is provided with an annular groove 166 which confronts the end of the stop shaft 160, and the end cap 26 is provided with a similar annular groove 168. Rotation of the shaft 32 translates the'discs v104 and 106 along the axis of the housing 10, and in like manner translates the shaft 160 along the axis of the housing 10. The closed end 14 of the housing 10 is provided with a bore 170 which extends into the groove 166, and a pin 172 isdisp'osed within the bore 170 and traverses the groove 166. In like manner, the end cap 26 is provided with a bore 174 which traverses the annular groove 168, and a pin 176 is disposed within the bore 174l and traverses the annular groove 168. In this manner, rotation of the shaft 32 in one direction causes the shaft 160 to enter into the annular groove 166 and engage the pin 172, thereby providing a stop in this direction; and rotation of the shaft 32 in the other direction causes the shaft 160 to be translated through the spool 72 into the groove 168 and abut the pin 176, thereby providing a stop for rotation in this direction also. The stopping action occurs as a result of shaft 160 acting in interference between pin 172 and the side of the adjacent aperture 162, when traversed by discs 104 and 106 to their extreme positions.' By keeping the separation between spool 68 and pin 172 to a minimum, the leverage ratio on shaft 160 becomes greater and hence the torque that the stop mechanism can withstand without damage is substantially increased.

The terminal board 18 178A, 178B, 178C, and nected to the turn of the end 14 of the housing 10,

are provided with the hooks 126 and is provided with four terminals 178D. Terminal 178A is concoi1.50A adjacent to the closed and terminal 178B is connected to the turn of thecoil 50A adjacent to the coil 50B. In like manner terminal 178C is connected to the turn of the coil 50B adjacent to the coil 50A, and the terminal the end cap 26. Thus the variable resistor here disclosed provides a 2-gang variable resistor with an adjustable tap on each resistor with terminals connected to the accommodate the stop shaft turn of the coil 50B adjacent to ends of each resistor and to the adjustable taps. lu making the end connections to coils 50A and 50B, the shaft 32 is rotated through the desired electrical angle and the proper turn of resistance wire located by searching for minimum resistance between various turns and the slider contacts 24a and 24b. The outer portions of coils 50A and 50B are accessible through slot 16 for making these connections. Intermediate fixed taps may be located in a similar manner, additional holes beingt provided at their corresponding positions in the housing 10.

It is clear that this variable resistor construction provides a multi-turn resistor with a contact displacement to resistance characteristic which achieves a high degree of linearity, since the resistance element is firmly cemented in place without undue distortion, and the individual resisance wire -turns are held in position by` the inner helix on both sides of the area engaged by the slider contacts. Further, a variable resistor may readily be constructed according to the teachings of this invention which is me chanically rugged and at a relatively low cost.

From the foregoing disclosure, the man skilled in the art will readily devise constructions and modifications of the disclosed construction which are within the scope of the present invention. It is therefore intended that the present invention be not limited by the foregoing disclosure, but rather only by the appended claims.

The invention claimed is:

l. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, said second helical coil having a smaller diameter than the first helical coil and the turns of said second coil confronting the space between two turns of the first helical coil, a disc rotatably disposed about the axis of the cavity having a peripheral ridge slidably disposed between adjacent turns of the second coil, and an electrical contact mounted to the disc and slidably abutting one of the turns of the Vfirst coil.

2. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, said second helical coil having a smaller diameter than the first helical coil and the turns of said second coil confronting the space between two turns of the first helical coil, a shaftrotatably and coaxially disposed within the cavity of the housing, a pair of spaced spools secured to the shaft, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rods, said disc having a protruding ridge disposed between two adjacent turns of the second helical coil, and an electrical contact mounted to the periphery of the disc aligned with the ridge thereof and slidably'l abutting one of the turns of the resistance coil.

3. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, a helical coil of resistance wire, mounted'coaxially within the cavity, said coil having spaced turns with electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, said second helical coil having a smaller diameter than therst helical coil and the turns of said second coil confronting the space between two turns of the first helical lcoil, a shaft rotatably, and coaxially disposed within the cavity of the confronting portions l housing, a pair of spaced spools secured to the shaft, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rod, said disc having a pair of protruding ridges disposed on opposite sides of one of the turns of the second helical coil, and an electrical contact mounted to the periphery of the disc onthe opposite side of the central aperture in the disc from the protruding ridges, said electrical contact slidably abutting one of the turns of the resistance coil.

4. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, said second helical coil having a smaller diameter than the first helical coil and the turns of said second coil confronting the space between two turns of the first helical coil, and an electrical contact constructed of resilient material having two generally parallel portions slidably abutting a common section of the resistance coil, each of said contact portions also abutting one of the pair of turns of the second coil confronting the section of the resistance coil.

5. A variable resistor comprising, in combination, a he sing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, said second helical coil having a smaller diameter than the first helical coil and the turns of said second coil confronting the space between two turns of the first helical coil, a shaft rotatably and coaxially disposed within the cavity of the housing, a pair of spaced spools secured to the shaft, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, a disc having a central aperture journallel about the shaft and a pair of bores on opposite sides of the aperture journalled about thc rods, said disc having a protruding ridge disposed between two adjacent turns of the second helical coil and a pair of protruding ridges on the opposite side of the disc disposed on opposite sides of one of the turns of the second helical coil, and an electrical Icontact constructed of resilient material mounted to the periphery of the disc adjacent to the single ridge, said contact having two generally parallel portions slidably abutting a common section of the resistance coil, each of said contact portions also abutting one of the pair of turns of the second coil confronting the section of said resistance coil.

6., A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, and walls disposed at the ends of the cavity having a central opening therein, a helical coil of electrically conducting resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, saidsecond helical coil having a smaller diameter than the first helical coil and the turns of said second coil confronting the space between two of the turns of the first helical coil, a shaft coaxially disposed within the cavity of the housing and journalled within the openings at the ends thereof, a pair of spaced spools secured to the shaft having aligned slots adjacent to the perimeter of the spools, a pair of rods secured to the spools and extending therebetween on opposite sides l of the shaft, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rods, said disc having a protruding ridge disposed between two adjacent turns of the second helical coil and a second aperture aligned with the slots in the spools, a second shaft secured within the second aperture of the disc and aligned with the slots in the spools, ani an electrical contact mounted to the periphery of the disc adjacent to the ridge thereof and slidably abutting one of the turns of the resistance coil.

7. A variable resistor comprising, in combinationya housing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, the turns of said second coil confronting the space between the turns of the first helical coil, a shaft rotatably and coaxially disposed within the cavity of the housing, a pair of spacedI spools secured to the shaft, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rods, said disc having a protruding ridge disposed between two adjacent turns of the second helical coil and a cutout extending therein from the periphery and intersecting each of the bores, said disc also having a slot extending inwardly from each of the cutouts, a leaf spring disposed in one of the slots and abutting the adjacent rod, and an electrical contact having one end disposed within the other slot and extending outwardly to abut the other rod, said contact being bent to conform to the periphery of the disc and having an outwardly extending hook at its opposite end slidably abutting one of the turns of the resistance coil.

8. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein, a helical coil of resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a second helical coil of electrically insulating material coaxially disposed within the cavity adjacent to the first coil, each portion of the turns of said second coil confronting the space between two turns of the first helical coil, a shaft rotatably and coaxially disposed within the cavity` of the housings a pair of spaced spools secured to the shaft, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, one of said rods having an axial slot, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rods, said disc having a protruding ridge disposed between two adjacent turns of the second helical coil and a pair of protruding `.ridges on the opposite side of the disc disposed on opposite sides of one of the turns of the second helical coil, an electrical contact mounted to the periphery of the disc adjacent to the protruding ridge and slidably abutting the first helical coil, an electrically conducting ring coaxially disposed within the cavity of the housing confronting the slot in one of the rods, and a second contact constructed of wire shaped electrically conducting spring material provided with a central loop, the loop of said contact being anchored within the slot in the rod and the ends of the contact slidably abutting the ring. g/

9. A variable resistor comprising, in combination, a housing having a cylindrical cavity therein and walls disposed at'the ends of the cavity having a central opening therein and annular grooves disposed coaxially about the opening confronting the cavity, a helical coil of electrically conducting resistance wire mounted coaxially within the cavity, said coil having spaced turns with confronting portions electrically insulated from each other, a secondhelical coil of electrically insulating material coaxially disposed within the cavity adjacent'to the first coil, the turns of the second coil confronting the space betwccn adjacent turns of the first helical coil, a shaft coaxially disposed within the cavity of the housing and journalled within the openings at the ends thereof, a pair of spaced spools secured to the shaft having aligned slots adjacent to the perimeter of the spools, a pair of rods secured to the spools and extending therebetween on opposite sides of the shaft, a disc having a central aperture journalled about the shaft and a pair of bores on opposite sides of the aperture journalled about the rods, said disc having a protruding ridge disposed between adjacent turns of the second helical coil and a second aperture aligned with the slots in the spools, a second shaft secured within the second aperture of the disc and aligned with the slots in the spools and the grooves in the walls of the housing, a

pin traversing each of the grooves in the walls of the housing adapted to abut the second shaft and form a rotational stop, and an electrical-contact mounted to the periphery of the disc adjacent to the ridge thereof and slideably abutting a portion of one of the turns of the resistance coil.

10. A` variable resistor comprising, in combination, a housing having a cylindrical cavity therein with a wall disposed at one end having an annular groove disposed therein coaxially relative to the cavity, a helical coil of electrically conducting resistance wire mounted coaxially withthe groove in the wall adapted to abut the second shaft and form a rotational stop for the support means.

References Cited in the file of this patent UNITED STATES PATENTS 2,371,159 Erb Mar. 13, 1945 2,519,752 Fox Aug. 22, 1950 2,595,189 Dewan Apr. 29. 1952. 2,747,061 Sorbet May 22, 1956 FOREIGN PATENTS 657,865 Germany Mar. 15, 1938 759,496 Germany Mar. 9, 1953 841,768 Germany June 19, 1952 

